Sunday, 17 June 2012

Digital Terrain Modelling and Mapping

This is the first post in a new blog series focusing on all the fun you can do with digital terrain data. It’s going to be more related to topographic than thematic mapping, although the two map types can be used in combination. I’ll also try to restrict myself to data and software in the public domain.

My focus area is going to be Jotunheimen in Norway, a mountainous area popular among hikers and climbers. The 29 highest mountains in Norway are all in Jotunheimen, including the very highest - Galdhøpiggen (2469 m). Here's a few pictures I took on a hike back in July 2006 which gives you a glimpse of the beautiful landscape:

It’s many different data formats for storing and working with digital terrain data. I’ll start looking at Digital Elevation Models (DEM) represented as rasters. A raster DEM is a grid of squares or pixels where each square represents an elevation at a geographic location. The size of each square (in meters or arc-seconds) tells us about the level of detail or data resolution.

In Norway, it’s possible to get digital terrain data with 10 meters resolution, but unfortunately it’s not yet available to the public. Instead I found a suitable dataset on Viewfinder Panoramas, created from topographic maps. You’ll find DEMs with global coverage at various resolutions here, here and here.

I downloaded two files from Viewfinder Panoramas (N61E007.hgt and N61E008.hgt), each covering an area of 1 degree latitude and longitude marked in yellow:

The original DEM data is unprojected and I’m projecting the data using UTM zone 32N (EPSG:32632). UTM coordinates (meters) are used to clip the dataset to an area covering 3,600 km² (60 x 60 km). The area is shown with a red square on the map above. The -r bilinear option is important when projecting elevation data because other resampling methods tend to produce odd stripes in the image.

We finally have a DEM for Jotunheimen (download). You won’t see much if you try to open this image in Windows. The image is almost all black. Use gdalinfo to see the properties of this image:

This image gives you a glimpse of the elevation in Jotunheimen. Dark areas are valleys with low altitude. Light areas are mountain ridges with high altitude. Areas with uniform shades are lakes or areas with less variation in elevation.

Hillshade is a more effective technique to visualise terrain data. That’s the topic for the next blog post.